The present invention relates to a method and apparatus for hot forming and hardening a workpiece such as a flat or preformed steel blank.
The following discussion of related art is provided to assist the reader in understanding the advantages of the invention, and is not to be construed as an admission that this related art is prior art to this invention.
In the field of vehicle construction, more and more vehicle parts made of high-strength and ultra-high-strength steel are being employed in order to be able to satisfy criteria for light-weight construction. This applies in particular to vehicle body construction where, in order to meet weight goals and safety requirements, i.a. structural and/or safety elements such as door impact beams, A and B columns, bumpers, side rails, and cross rails, are increasingly produced from hot formed and press-hardened steel having tensile strengths greater than 1000 MPa.
Published German patent document DE 24 52 486 C2 describes a method for press-shaping and hardening a steel sheet that is relatively thin and has good dimensional stability. A sheet made of boron-alloyed steel is heated to a temperature above its upper Ac3 transformation point in the iron carbon diagram (hereinafter referred to as “I-C-D”) and then in less than 5 seconds is pressed into the final shape between two indirectly cooled tools that change its shape significantly, and, while still in the press is subjected to rapid cooling such that a martensitic or bainitic structure is obtained. Using these measures produces a product which has good shape accuracy, good dimensional stability, and high strength, and which is well suited for structural and safety elements in vehicle construction. This process is hereinafter referred to as hot forming and press-hardening. Both preformed parts as well as flat blanks can be hot formed and press-hardened. In preformed parts, the forming process can also be limited to a shaping of a small percentage of the final geometry or to calibration.
Different applications in the automobile industry require the production of formed parts of high strength in certain regions while having a comparably higher ductility in other regions. In addition to reinforcing with additional metal sheets or joining parts that have different strengths, it is also known to heat-treat a formed part in such way as to exhibit local regions of higher strength or higher ductility.
Published U.S. patent document US 2004/0060623 describes a method of producing a hardened metal part having at least two regions with different ductility. A flat or preformed blank is heated to an austenitization temperature in a heating device and then transported along a transport path to a hardening process. During transport, first regions of the flat or preformed blank that have higher ductility properties in the final part are cooled. The method is optimized for mass production by quenching the first regions from a predetermined cooling start temperature that is greater than the γ-α transformation temperature in the I-C-D, and by terminating quenching when a predetermined cool stop temperature is attained before any transformation into ferrite and/or perlite has occurred or after only a slight transformation into ferrite and/or perlite has taken place. Then, the workpiece is maintained approximately under isothermal condition for converting the austenite to ferrite and/or perlite, while the hardening temperature in second regions which have comparably lower ductility properties in the final product, is kept just high enough for sufficient martensite formation in the second regions during a hardening process. Thereafter, the hardening process is performed. In this method, more thermal energy is added to the first regions of the flat or preformed blank than is necessary, and thermal energy is removed in a second process step, which also consumes energy. The method therefore has a relatively poor energy balance.
German Pat. No. DE 101 08 926 C1 discloses a thermal treatment process for changing the physical properties of a metal article. The article is irradiated, at least in a predetermined surface section, with electromagnetic radiation from an emitter having a radiator temperature of 2,900 K or more in the near infrared range with a high power density. As a result, the material of a surface layer is heated to a predetermined treatment temperature in dependence on material parameters. Then the irradiated surface region is actively cooled and thus hardened and tempered. However, completely heating an article that has a large surface area from room temperature to hardening temperature using this method would be too uneconomical for an industrial hot forming line.
U.S. Pat. No. 7,540,993 discloses a method for producing a formed part that has at least two regions with different ductility from a semifinished product made of hardenable steel by heating in a continuous furnace followed by a hardening process. During transport through a continuous furnace, the semifinished product to be heated simultaneously passes through at least two zones in the continuous furnace that are adjacent one another in the travel direction and that have different temperature levels and thus are heated differently so that in a subsequent hardening process at least two structural regions are created that have different ductility. Both zones are separated from one another by a partition such that a workpiece passing through the furnace has parts in both zones so separate temperature control is possible in each zone. However, this multizone furnace is a special furnace for parts that are to be heated zone-wise.
It would therefore be desirable and advantageous to provide an improved method and apparatus for making a blank with differently hardened regions in a hot forming line to obviate prior art shortcomings and to realize a press cycle which is as economical as possible.